Keeping portability in mind, consumer electronic devices today are designed. This design paradigm leads to smaller speakers in all portable electronic devices. The speaker having smaller physical dimension severely limits sound reproduction, especially in the low frequency registers, which leads the consumer to be dissatisfied with the sound output quality thereof. A conventional solution to this problem is to amplify the low frequency component of an audio input signal. However, this solution, increasing the energy level of the low frequency component, not only leads to extra power consumption, but also makes the speaker damage as well.
A better solution to improve the reproduction performance without boosting low frequency component is to utilize the psychoacoustic technique. The psychoacoustic technique demonstrates the existence of a phenomenon in harmonics known as “virtual pitch”, in which the brain senses a frequency having the greatest common factor in harmonic frequencies, even if the amplitude of the fundamental frequency is zero. To illustrate, if the ear hears a tone having overtones with frequencies of 600, 800, 1000 and 1200 Hz, the pitch will nearly be identified as that of a 200 Hz tone.
A scheme in the prior art disclosed in U.S. Pat. No. 5,668,885 provides a low frequency audio conversion circuit. Another scheme in the prior art disclosed in U.S. Pat. No. 5,930,373 provides a psychoacoustic alternative signal generator; the focus of this patent is volume equalization; this means the loudness of the residue harmonic signal is brought to match the loudness of the low frequency signal. A further scheme in the prior art disclosed in U.S. Pat. No. 6,134,330 provides an enhancing device. A further scheme in the prior art disclosed in U.S. published patent No. 2006/0159283 provides a bass-enhancement circuit.
In practice, these audio systems often require the filter with low complexity in order to easily implement, and the speaker often shows the property of the cutoff frequency. The property causes the speaker difficult to reproduce the signal in the range out of the cutoff frequency. If the filter (e.g. a band pass filter) with the low complexity (low cost, in other word) is employed to implement, the original input signal still exists because the transition bandwidth is wide and the stop band is not approach to zero. The output energy is wasted because the output signal, processed by the band pass filter, still includes the original low frequency input signal which cannot be reproduced by the loudspeaker.
There is another drawback on the existence of the low frequency signal. A compressor is required in the system to avoid the levels of the output signal of the audio system going beyond the threshold of the loudspeaker. The compressor compresses the bass-enhancement signal back down below the threshold, wherein the compressor compresses not only the low frequency part of the bass-enhancement signal but also the high frequency part of the bass-enhancement signal. For instance, the output signal of the compressor has the high frequency part of the acoustic wave. When the output high frequency part is compared with the original high frequency part, the intensity of the output signal is much lower than that of the input signal. That is to say, the overall levels of the output signal increases when the low frequency part is present, which causes the compressor gain to be further decreased in order to avoid going beyond the threshold; thus, the high frequency part of the output signal of the compressor with the low frequency part is decreased more than that without the low frequency part.
Besides, the conventional method further has a drawback described as follows. In order to avoid the levels of the bass-enhancement signal going beyond the threshold of the speaker, the bass-enhancement signal (i.e. the gain is less than 1) is compressed. The whole of the bass-enhancement signal is compressed to be lower than the threshold, which causes the high frequency part of the bass-enhancement signal is compressed a lot and the output dynamic range of the bass-enhancement signal is also compressed.